The present invention is related to composite materials exhibiting current limiting behavior and the ability to recover rapidly from fault current events.
Fault currents are large (usually temporary) increases in the normal operating current flowing in a power transmission system. A fault current can occur from any number of different events including lightning strikes or catastrophic failure of electrical equipment which can cause short circuits. A short circuit, for example, can cause a twenty-fold or more increase in current flowing through the circuit.
Conventional circuit breakers are used in virtually every power transmission and distribution system to xe2x80x9copenxe2x80x9d the circuit and interrupt current flow in the event of a fault. The fault current level grows as new equipment is added over time. However, with an increase in the magnitude of fault current comes an increase in the size and expense of the circuit breaker. Moreover, conventional circuit breakers do not open instantaneously. The fault current is generally first detected by a current sensor which generates a signal to a control circuit. The control circuit processes the signal and then generates a control signal to open the circuit breaker. During these steps (which may have a duration as long as 50-2000 msec or more), the circuit breaker, as well as other parts of the transmission system, are subjected to the higher fault current level. Thus, the circuit breaker, transformers and other components of the system are often rated to withstand the higher current levels for a period of time.
Fault current limiters were developed to insert impedance in a connection quickly so as to reduce the magnitude of the fault current, thereby protecting the circuit breaker and the power transmission system. Many fault current limiters include tuned reactance circuits which store energy in proportion to the circuit inductance.
Often a circuit breaker will be designed to automatically reclose a short time after it opens, in case a transient fault has cleared. It is therefore desirable for a fault current limiter to exhibit fast recovery characteristics, so that it will function when the circuit breaker recloses.
An important property of a superconductor is the disappearance of its electrical resistance when it is cooled below a critical temperature Tc. For a given magnetic field and temperature below Tc, there exists a characteristic critical current Ic, below which the superconductor will exhibit virtually no resistance. Above Ic, the resistance (and electric field) increase very rapidly. If the superconductor is embedded or co-wound with a conductive matrix, current is divided between the superconductor and the matrix as a parallel circuit. (Below Ic, the superconductor carries substantially all of the current because of its zero resistance).
The transition characteristics of superconducting materials have been used advantageously to develop superconducting fault current limiters. For example, in one conventional approach, a superconducting current limiting device is constructed using a thin coating of superconducting material deposited onto a surface of a heat-dissipating wafer (e.g., sapphire). When a fault is detected, the coating transitions into its normal state and becomes resistive, thereby limiting the flow of current until a circuit breaker, in series with the device, interrupts the current flow. In other approaches, bulk superconducting rods or rings are used in devices which inductively limit the level of fault currents.
Present fault current limiters, whether they be stand alone devices or integrated into the devices which they are intended to protect, consist of a plurality of discrete electrical components. An elongated composite material which inherently possesses current limiting capability would enable simpler designs integrating fault current limiting functionality. For instance, such a conductor could be incorporated in the windings of a transformer resulting in a fault current limiting transformer. This concept defines a need for a current limiting elongated composite material that exhibits a desirable electrical field in the presence of a fault current that is greater than the steady state peak current.
In one aspect, the invention comprises an elongated current limiting composite, comprising an oxide superconducting member and a second electrically conductive member substantially surrounding the oxide superconducting member. The composite has fault limiting properties as follows: when a current of about 3-10 times the operating current is passed through the composite, it exhibits an electric field of about 0.05-0.5 V/cm. The operating current is between about one half the critical current and about the critical current at a selected operating temperature, the operating temperature being less than the critical temperature of the superconductor.
The electrically conductive member may be a silver-containing matrix, which may further include gallium, tin, cadmium, zinc, indium, and antimony. It may also comprise a bonding agent such as an adhesive or solder, which may bond a thermal stabilizing element to the matrix. The thermal stabilizing element may comprise, for example, stainless steel or a copper alloy. The composite may be in the form of a wire.
In some embodiments, the heat capacity of the composite is selected to be sufficient to prevent the composite temperature from rising above the critical temperature of the at least one oxide superconducting member during a fault event. This heat capacity may be calculated according to Equation (2), infra, for a fault lasting 50, 150, 250, 500, 1000, or 2000 msec. In other embodiments, the composite may be configured so that sufficient heat can be dissipated from the composite after a fault event to allow the composite to cool to the operating temperature. This configuration may be determined with reference to Equation (3), infra, for a fault lasting 50, 150, 250, 500, 1000, or 2000 msec. For still other embodiments, both Equation (2) and Equation (3) may be satisfied for faults lasting 50, 150, 250, 500, 1000, or 2000 msec.
In other aspects, the invention comprises a current-limiting transformer, having a composite as described above in electrical series with its windings. The transformer may further comprise integrated cooling means for holding the transformer at the operating temperature. The invention further comprises a current limiter comprising the composite described above and integrated cooling means for holding the composite at the operating temperature.
In still other aspects, the invention comprises a method of limiting current during a fault event in a power transmission system carrying an operating current. The method comprises interposing a superconducting composite comprising a superconducting oxide member substantially surrounded by a second conductor, where the composite can carry the operating current of at least half the critical current with a voltage gradient of less than 1 xcexcV/cm in the absence of a fault, and exhibits a voltage gradient in the range of 0.05-0.5 V/cm in the presence of a fault in which the system carries a current of about 3-10 times the operating current.
The second conductor may be a silver-containing matrix, which may further comprise gallium, tin, cadmium, zinc, indium, or antimony. It may also comprise a bonding agent such as adhesive or solder, which may act to bond a thermal stabilizing element to the matrix. This thermal stabilizing element may be, for example, stainless steel or a copper alloy. The composite may be configured in the form of a wire, and may also be configured so that it does not rise above the critical temperature of the superconductor during a fault event.
The composite may further be placed in electrical series relation with a circuit breaker which interrupts current in response to a fault event, for example after a period of 50-2000 msec, a period of 100-1000 msec, or a period of 200-500 msec.
Unless otherwise noted, xe2x80x9cmatrix resistivityxe2x80x9d refers to bulk resistivity of the matrix which is determined across the many grains of the matrix material along the wire axis, while xe2x80x9ccomposite resistivityxe2x80x9d refers to the resistivity of the composite article, including superconducting element(s), matrix, and any additional laminated components.
By xe2x80x9coperating current densityxe2x80x9d of a superconducting composite as that term is used herein, it is meant the total current passing through both superconductor and matrix of the composite, divided by the cross-sectional area of the composite, including superconducting oxide filament(s), matrix, and any other components through which current may pass. This quantity is denoted by Jop.
By xe2x80x9cengineering critical current densityxe2x80x9d as that term is used herein, it is meant the total critical current of the superconducting members of a superconducting composite, divided by the cross-sectional area of the entire composite, including both superconducting oxide filaments and conductive matrix. This quantity is denoted by Je.
By xe2x80x9ccritical currentxe2x80x9d as that term is used herein, it is meant that the current at which there is a dissipation of 1 xcexcV/cm for DC applications, or 1 mW/A-m for AC applications.